1. Field of the Invention
The present invention relates generally to an improved method of analyzing petroleum products in particle form. More particularly, the present invention relates to an improved method for analyzing the quality of petroleum by-products in particle form, especially coke, and relates to improved methods for refining petroleum to obtain desirable coke by-products.
2. Prior Art
As will be appreciated by those skilled in the art, petroleum products are analyzed for many purposes by using various tests. The test results are used for many purposes, sometimes even including controlling the refining of these products in order to achieve desired results but more often for appropriately classifying the products for subsequent sale.
For example, coke is a resulting by-product from petroleum refining that must be appropriately classified prior to sale. Cokes are typically classified by measuring their coefficient of thermal expansion (CTE) which provides the rate of expansion (or contraction) of a substance with temperature change.
In one use, coke is used in manufacture of large graphite electrodes for electric arc furnaces employed in the steel industry. It is known that cokes with lower CTE (e.g. FCTE=0.0-2.0.times.10.sup.-7 /.degree. C.) result in better performance of graphite electrodes under electric arc furnace (EAF) operations than those with higher CTE (e.g. FCTE&gt;4.0.times.10.sup.-7.degree. C.). Therefore, lower CTE cokes are more desirable than higher CTE cokes.
It is believed that, in typical representative coke samples, the lower CTE materials consist of a larger number (percent) of highly needle-like particles than the higher CTE materials. Needle-like particles are those whose structure has a preferential orientation. They are generally elongated as a result; hence the term "needle". For a particular premium coke grade, the needle-like structure is not constant throughout the coke particles of that grade. Instead, there is a distribution of highly needle-like to highly non-needle-like particles.
One known method of classifying coke involves testing or examining these calcined cokes to determine the coke CTE. This procedure involves extrusion of a mixture of calcined coke particles and a binder pitch followed by baking, graphitizing and measurement of CTE of the resulting artifacts. Typically, the artifact is heated and measurements are taken at various temperatures to determine the CTE for a particular batch of coke. As will be appreciated by those skilled in the art, this procedure is fairly involved and time-consuming, on the magnitude of three to four days for a CTE measurement.
The resulting CTE determination has been previously used to segregate cokes into quality grades. However, the known methods for measuring coke CTE are complex and time-consuming. Thus, an improved method for classifying cokes is desirable.
It has been observed that, to even an untrained eye, higher quality cokes are shinier than lower quality ones. Subsequent research has shown (KOA Oil Company, 1997 Carbon Conference, PSU) that there is a correlation between CTE measurements and the "shininess" of a field of coke particles.
Another method of classifying coke is to use the varying reflectivities of the coke. The lustre method of the present invention quantifies this visual perception by digitally measuring the reflection of light from a pan of coke particles. This measurement is the coke's lustre (hereinafter, the phrase "coke lustre" will be used to refer to the intensity of visible light reflected from the surface of a coke particle or a layer of coke particles). The measurement of coke lustre is approximately analogous to extracting and counting the particles with the most needle-like character. Thus, coke lustre reveals information that can be used for ranking cokes based on their needle-like structure that is similar to CTE measuring methods.
Measuring coke lustre is easier than other known methods of physically measuring coke CTE. Classification of cokes based on lustre measurements has been difficult because only small differences in lustre are seen, even when CTE differences are significant. In other words, it has been difficult to correlate the small deviations noticed between coke lustres with a meaningful correlation with CTE variances.
Therefore, a need exists in the art for an improved method of classifying coke. A particularly desirable improvement would be an improved method of classifying coke based on its reflectivity or lustre as opposed to existing methods of physically measuring coke CTE. An even more desirable improvement would be a method for increasing the accuracy and reliability of such a lustre classification method.